Perpendicular magnetic recording (PMR) has become the mainstream technology for disk drive applications beyond 200 Gbit/in2, replacing longitudinal magnetic recording (LMR) devices. Due to the continuing reduction of transducer size, high moment soft magnetic thin films with a Bs above 22 kG are required for write head applications. PMR uses a magnetic yoke surrounded by field coils that terminates in a write pole which serves as the write head. The write pole must be wide enough at one end to attach to the yoke and narrow enough at the other end to confine the write flux to a very small area typically about 0.1×0.1 microns at an air bearing surface (ABS). The write pole is adjoined on the sides by a dielectric layer (side gap) and has a top surface including a trailing edge that is covered with a write gap to separate the write pole from a trailing shield.
A conventional PMR write head as depicted in FIG. 1 typically has a main pole layer 10 or write pole with a pole tip 10t at an ABS 5 and a flux return pole (opposing pole) 8 which is magnetically coupled to the write pole through a trailing shield 7. Magnetic flux in the write pole layer 10 is generated by coils 6 and passes through the pole tip into a magnetic recording media 4 and then back to the write head by entering the flux return pole 8. The write pole concentrates magnetic flux so that the magnetic field in the write pole tip 10t at the ABS is high enough to switch magnetizations in the recording media 4. A trailing shield is added to improve the field gradient in the down-track direction.
Referring to FIG. 2, a top view is shown of a typical main pole layer 10 that has a large, wide portion called a yoke 10m and a narrow rectangular portion 10p called a write pole that extends a neck height (NH) distance y from the ABS plane 5-5 to a plane 3-3 parallel to the ABS where the write pole intersects the yoke at the neck 12. The main pole layer 10 flares outward at an angle θ from a dashed line 11 that is an extension of one of the long rectangular sides of the write pole 10p. PMR technologies usually require the write pole 10p at the ABS to have a beveled shape (as viewed from the ABS) so that the skew related writing errors can be suppressed. In other words, the top edge 10a of the main pole layer 10 usually overhangs the lower edge 10b by a certain amount.
Although a PMR head which combines the features of a single pole writer and a soft magnetic underlayer has a great advantage over LMR in providing higher write field, better read back signal, and potentially much higher areal density, PMR still faces some challenges. One issue is related to the manufacture of PMR heads wherein some designs require an alumina layer around the main pole to be removed in order to allow a conformal side gap to be formed along the sides of the trapezoidal shaped write pole prior to write gap deposition. One problem associated with the conventional alumina etching process is the potential for main pole corrosion during de-ionized water soaking steps as predicted thermodynamically in “Atlas of electrochemical equilibria in aqueous solutions” by M. Pourbaix, 2nd English Ed., 1974, Houston, Tex., National Association of Corrosion Engineers. Main pole corrosion lowers production yields and degrades product quality but the root cause of corrosion is not fully understood. A possible corrosion mechanism may be due to an oxidation-reduction reaction associated with the following two equations (1a) and (1b):Reduction: O2+2H20+4e−=>4OH−  (1a)Oxidation: Fe=>Fe2++2e−  (1b)
Another problem with the typical alumina etching process is an unstable alumina etching rate which tends to drift lower because of the absorption of CO2 from air in the alkaline EDTA etch solution and an insufficient buffer capacity in the EDTA solution. As a result, undesirable alumina residue builds up around the main pole thereby affecting product performance.
A routine search of the prior art revealed the following references. In U.S. Pat. No. 7,748,104, a method of preventing corrosion during formation of the write pole is disclosed and involves forming a gap layer comprised of a noble metal on the write pole. However, this method does not address the susceptibility of the write pole to corrosion prior to formation of the gap layer.
U.S. Pat. No. 7,536,775 discloses a method of selectively removing alumina around a main pole layer to avoid damaging the main pole. In particular, an aqueous solution of sodium carbonate or sodium borate is used at 50° C. and is then followed by a water rinse. The method does not teach how to improve etch uniformity or reduce residues.
In U.S. Patent Application Pub. No. 2010/0006860, N2 is bubbled into a hot DI water solution to oxidize a top surface of a SiC substrate and thereby form a gate oxide.
U.S. Patent Application Pub. No. 2008/0253035 describes alumina etching with an alkaline solution comprising NaOH or KOH with EDTA at a pH greater than about 10.5 at 80° C. for about 15 seconds. However, none of the issues including corrosion, etch uniformity, or etch residues are addressed.
Thus, an improved alumina etching process is needed to overcome main pole corrosion and alumina residue issues regularly encountered in current fabrication techniques.